1. Field of the Invention
The present invention relates to a picture processing apparatus and a processing method thereof for performing an interpolating process in such a manner that the interlace relation of an original picture is kept in performing a line number converting process with a field memory for accessing an interlace signal for each line.
2. Description of the Related Art
Normally, a video signal is sent and processed in an interlace system. As is known, in a video signal corresponding to the interlace system, one frame is composed of two fields that are an odd field and an even field. Lines that are skipped in the odd field are scanned in the even field. In this case, the scanning start position of the odd field is different from the scanning start position of the even field by 0.5 H (horizontal frequency) as timing difference. The timing difference keeps the interlace relation.
On the other hand, there are needs for enlargement and reduction of pictures. A picture is enlarged or reduced by a line number converting process for a video signal. The line number converting process is performed by writing a video signal to the field memory line by line (every 1 H). Since the video signal is written line by line, the odd field and the even field are written to the field memory in the same manner. In other words, a particular line of each of the odd field and even field is written to the same address of the field memory. Thus, the deviation of the scanning start position by 0.5 H does not take place. Consequently, in the space of the field memory, the above-described interlace relation does not become kept.
Thus, when the picture enlarging process for increasing the number of lines in the fields is performed by reading data from the field memory, the resultant resolution adversely deteriorates. This is because when the number of lines is converted by linearly interpolating adjacent two lines in the field, the variation of the relation of lines in each field adversely affects the interpolating process. However, when the picture reducing process for decreasing the number of lines in the fields is performed, such a problem does not take place.
Thus, conventionally, when a line is read from the field memory, the read timing between the fields is varied for the interlace time period (namely, 0.5 H). Thus, the problem of the deterioration of the resolution in the enlarging process was solved. This method is effective when the number of lines is increased an integer number of times (for example, two times).
FIGS. 7A to 7C show examples of which lines are read at different read timings that vary for 0.5 H and linearly interpolated so as to increase the number of lines two times. Pixels in FIGS. 7A to 7C show representative points of which the horizontal positions on individual lines are the same. This representation applies to other drawings that follow. ".largecircle." represents a pixel in white level. "X" represents a pixel in black level. Pixels on lines of the odd field and even field are interlaced. ".circle-solid." represents a dirk gray whose brightness is 50% or less. Hatched ".largecircle." represents a bright gray whose brightness is 50% or more. In FIGS. 7A to 7C, the top odd line is for example the firs line.
An original signal shown in FIG. 7A is written to the field memory in such a manner that the original signal in the odd field is treated in the same manner as the original signal in the even field as shown in FIG. 7B. When the number of lines is converted and increased two times, the interpolating process is performed at positions denoted by arrows corresponding to these pixels as shown in FIG. 7B. The interpolating process is performed every 1/2 lines in each field. When an original signal is read, timing for 0.5 H is controlled. Thus, pixels as shown in FIG. 7C are obtained. A pixel that is interpolated with a pixel of white level and a pixel of black level becomes a gray pixel. Thus, in the converting process for increasing the number of lines two times, the interpolating process can be properly performed in the conventional method.
In the converting process for increasing the number of lines two times, there are several methods. As a first method, one frame image which the odd field and the even field are overlaid is displayed in succession over two fields without an in-field interpolating process. As a second method, each field is read twice so as to increase the number of lines. However, in these methods, in the first method, as shown in FIG. 8A, since pictures that have a delay each other are displayed on the same screen, the motion thereof becomes unnatural. In the second method, as shown in FIG. 8B, the sufficient resolution deteriorates. Thus, it cannot be said that such methods are good.
On the other hand, there are needs for a variable enlarging process rather than a fixed enlarging process (for example, the enlargement ratio is not an integer value such as 4/3 times the original picture). In this case, the optimum interlace relation cannot be maintained. Thus, the resolution will deteriorate or lines will flicker. Consequently, the picture quality of the resultant picture becomes poor.
FIGS. 9A to 9C show examples of the result of a conventional interpolating process in the case that an enlarging process for 4/3 times was performed. As shown in FIG. 9B, the interlace relation of the odd field and even field is lost in an original pixel signal (FIG. 9A) written to a field memory. The resultant signal is written to the field memory. When the number of lines is converted and increased 4/3 times, the interpolating process is performed at the positions denoted by arrows shown in FIG. 9B. In other words, the interpolating process is performed every 1/(4/3) lines in each field (namely, every 3/4 lines). When data is read, timing for 0.5 H is controlled. Thus, pixels as shown in FIG. 9C are obtained. Consequently, since the symmetrical shape of the original pixel signal becomes asymmetrical, the resultant signal is observed as lines that flicker.
Thus, in the conventional method, when data is written to the field memory, the interlace relation is not kept. Consequently, when a picture is enlarged with an enlargement ratio of a non-integer value (such as 4/3 times), the resultant picture gets distorted against the original picture.